In order to produce oil and gas from reservoir, oil and gas have to pass through a sufficiently permeable flow path to the wellbore. For formation with low permeability, stimulation such as fracturing is needed to increase the permeability and increases the oil and gas production. Fracturing fluids are pumped at high pressure and rate into the reservoir interval to be treated, causing a vertical fracture to open. The wings of the fracture extend away from the wellbore in opposing directions according to the natural stresses within the formation. Proppant, such as grains of sand of a particular size, is mixed with the treatment fluid to keep the fracture open when the treatment is complete. Hydraulic fracturing creates high-conductivity communication with a large area of formation and bypasses any damage that may exist in the near-wellbore area. Therefore, it is important for the fluid to be viscoelastic to suspend and carry the proppant into the fracture.
Water soluble polymers such as guar and derivatives are commonly used to thicken the fluids. However, the polymer has residue left in the reservoir, which will damage the formation and affect the production rate.
Viscoelastic surfactant (VES) has been used in oil and gas applications as fracturing fluid. The viscoelastic surfactant forms wormlike micelles with self-assemble of surfactant, and gives viscoelastic properties. Unlike polymer thickener, viscoelastic surfactant based fluids do not reduce the permeability as no resides are left. In addition, it is simple to operate with fewer additives in the fields. No cross linking agent and breaker are needed for VES. Viscosity of the fluid is reduced upon exposure to crude oil, and therefore having better fracture clean up.
Viscoelastic surfactant is usually made of certain amount of suitable surfactants such as anionic, cationic, nonionic and zwitterionic surfactants. When concentration of surfactant exceeds a critical concentration, surfactant molecular aggregates into elongated micelle, which can interact to form a polymer like network exhibiting elastic behavior.
Cationic surfactant such as long chain quaternary ammonium salts have been used as fracturing fluids. Salts such as potassium chloride, ammonium chloride and sodium salicylate have been used to generate viscoelasticity. U.S. Pat. No. 5,551,516, U.S. Pat. No. 5,964,295, U.S. Pat. No. 6,435,277, and U.S. Pat. No. 6,412,561 were disclosed cationic surfactant to give viscoelasticity. Amphoteric/zwitterionic surfactants in mixture of organic acid, salts and cationic surfactants were also used to generate viscoelasticity which is described in U.S. Pat. No. 6,703,352, and U.S. Pat. No. 8,022,016.
The viscoelastic surfactant is typically used in concentration of 2˜6%, which is much higher than that of a typical polymeric thickener at <0.6%. The high usage concentration of viscoelastic surfactant increases the cost of such materials and inhibits its widely application. It would be desirable to have VES fluids that could be formed and effective at lower usage concentration.